The studies are designed to provide basic information concerning the transepithelial transport pathways and mechanisms by which water and NaCl are absorbed in the mammalian proximal convoluted tubule and the ways in which these processes may be altered by varying physiological and pathological states. The studies are divided into four projects and will be performed using the isolated, perfused rabbit tubule technique. The first project will examine the cellular mechanism of electroneutral, transcellular NaCl absorption. The apical and basolateral transport mechanisms, eg. NaCl and KCl symporters and Na/H and Cl/HCO3 exchangers, will be specifically investigated. The second project will examine the mechanism whereby peritubular protein concentration modulates electroneutral, transcellular NaCl absorption. Further characterization of the protein effect will be addressed by investigating its colloidal specificity, its charge specificity, its effect on cell volume and its effect on intracellular chloride ion activity. The third project will examine the relative contribution of the transcellular and the transjunctional transport pathways to transepithelial water absorption. Differentiation among water movement through large junctional pores, small cell membrane pores and membrane lipid will be made by defining the unstirred layer-corrected osmotic to diffusive water permeability ratio, by determining the apparent activation energies for unstirred layer-corrected diffusive and osmotic water permeability, and by re-examination of the NaCl reflection coefficient. A new fluorescence method will be developed to measure osmotic water permeability and the reflection coefficient for NaCl. The forth project will examine the electrical characteristics of acidification. In particular, the means by which electroneutral acidification influences the organic solute coupled sodium transport potential difference will be examined, placing special emphasis on acidification-induced changes in transepithelial resistance and transepithelial current flow.